This invention relates to a wheel cylinder and, more particularly, to a wheel cylinder including a piston for actuating a drum-style brake for vehicles, such as automobiles.
Drum-style brakes for vehicles are well known and include a wheel cylinder usually having two hydraulically actuated pistons connected to corresponding brake shoes, which face the interior circumferential surface of the drum. The pistons respond to increases in hydraulic pressure caused by the actuation of the brake pedal and expand outwardly from the wheel cylinder body in opposite directions to displace the shoes outwardly into frictional contact with the brake drum. An arrangement of springs, or the like, is utilized to return the shoes, and therefore the pistons, to their original positions once the hydraulic pressure is reduced.
Typically the pistons and housing are fabricated from cast iron or other relatively soft material. Therefore, the portion of the pistons engaging the brake shoes must be fabricated from a stronger material, such as forged steel, which can withstand the forces from the brake shoe or a stronger component, usually in the form of an insert, has to be manufactured separately and assembled to each piston, which adds to the manufacturing and assembly labor and to the cost of the pistons. Another problem with these designs concerns the durability of the wheel cylinders. Typically, the wheel cylinder is located at a relatively low point in the vehicle. With the weight density of water being higher than that of brake fluid, water in the brake system will migrate, over time, toward the lowest point in the system where the wheel cylinders are located causing rust problems of the steel or iron components.
Although some more recent designs of wheel cylinders of this type include a body and/or piston of aluminum, these designs still require a separate insert of a different, stronger, material for engaging the brake shoe.